In recent years, the USB architecture has become a widely employed high-speed serial cable bus designed that supports data exchange between a host computer and a wide range of simultaneously accessible peripherals. The attached peripherals share USB bandwidth through a host-scheduled, token-based protocol. More specifically, the USB 3.0 architecture, specified in the USB 3.0 Specification, Revision 1.0, Nov. 12, 2008, managed and disseminated by the USB Implementers Forum, Inc., includes highly desirable features over previous USB architectures, including the SuperSpeed protocol.
According to the USB 3.0 Specification, a USB port transmitter can transmit various types of transmissions, including link commands, header packets, data packets, SKP ordered sets, Training Sequence (TS) ordered sets, and Bit Error Rate Test (BERT) ordered sets. USB devices transmit these transmissions to one another on serial links that connect ports of the USB devices. When multiple link command, header packet, data packet, and SKP ordered set transmissions are scheduled to be transmitted by a USB port in the U0 operational state (the normal operational state), an arbiter is needed to be in charge of prioritizing the transmissions to be transmitted on the USB link to which the port is connected. Sections 10.7.5 and 10.7.9 of the USB 3.0 Specification include FIGS. 10-16 and 10-18, respectively, which describe the functional requirements of a USB port transmitter state machine. From these Figures, a priority rule of scheduled transmissions that may be inferred from these sections of the USB 3.0 Specification is as follows:                1. Link Command transmissions (All Link Commands) and Skip Ordered Set (if required)—highest priority        2. Header Packet transmission (Transaction Packet (TP), Link Management Packet (LMP), and Isochronous Timestamp Packet (ITP)) and Skip Ordered Set (if required)—next highest priority        3. Data Packet transmissions and Skip Ordered Set—lowest priority        
The various types of transmissions referenced above are defined in the USB 3.0 Specification in the Terms and Abbreviations section on pages 2-1 to 2-7.
A USB port arbiter that determines which scheduled transmission to transmit first based on the single priority rule inferred from the USB 3.0 Specification may not exhibit good performance and transmission behavior in some circumstances.